Phenol and cyclohexanone are important products in the chemical industry and are useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, a common route for the production of phenol is the three-step Hock process via cumene. In the first step of the process benzene is alkylated with propylene in the presence of an acidic catalyst to produce cumene. The second step comprises oxidation, preferably aerobic oxidation, of cumene to the corresponding cumene hydroperoxide. The third step comprises cleavage of the cumene hydroperoxide, usually in the presence of a sulfuric acid catalyst, into substantially equimolar amounts of phenol and acetone, a co-product.
It is known that phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt and mixtures thereof. This patent reference also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
The process for making phenol from cyclohexylbenzene differs from the cumene process in several respects. Firstly, oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide is much more difficult than oxidation of cumene and requires elevated temperatures and the use of a catalyst, such as N-hydroxyphthalimide (NHPI). As a result, the cyclohexylbenzene oxidation effluent is also generally at elevated temperatures so that cooling this stream back to ambient temperature would incur additional operating cost. Also, in view of the high boiling point of cyclohexylbenzene, concentration of the cyclohexylbenzene hydroperoxide by evaporation of the residual cyclohexylbenzene is much more difficult. In addition, the cleavage chemistry for cyclohexylbenzene hydroperoxide is much more complicated than that for cumene hydroperoxide, particularly since more routes for by-product formation exist with cyclohexylbenzene hydroperoxide. Moreover, cyclohexanone is much more prone to acid-catalyzed aldol condensation reactions than acetone so that significant yield loss is possible unless the cyclohexylbenzene hydroperoxide cleavage is closely controlled.
There are other disadvantages of using sulfuric acid for cyclohexylbenzene hydroperoxide cleavage: 1) sulfuric acid is corrosive, especially in the presence of water, requiring expensive materials for reactor construction; 2) sulfuric acid needs to be neutralized before product separation and distillation, which requires additional chemicals such as phenate, caustics, or organic amines; and 3) the salt generated from neutralization requires separation and disposal and the waste water needs to be treated. Therefore, there are strong incentives to replace sulfuric acid with a heterogeneous cleavage catalyst that eliminates these drawbacks.
In addition, International Patent Publication No. WO2012/145031 discloses that large pore zeolites of the FAU type having a unit cell size of less than 24.50 {acute over (Å)} exhibit a unique combination of high activity and high selectivity activity for the conversion of cyclohexylbenzene hydroperoxide into phenol and cyclohexanone.
Although a number of solid acids have shown promise as catalysts for the cleavage of cyclohexylbenzene hydroperoxide, to date their utility has been limited because of their tendency to undergo rapid deactivation. According to the present invention, it has now been found that the cycle life of solid acid catalysts used in the cleavage of cyclohexylbenzene hydroperoxide into phenol and cyclohexanone can be increased if the concentration of cyclohexylbenzene hydroperoxide in the effluent from the upstream oxidation process is increased before the effluent is supplied to the cleavage reaction.